1. Field of the Invention
The present invention relates to a document reading apparatus, and more particularly, to a document reading apparatus that reads a document image with a CCD image sensor.
2. Description of the Related Art
Conventionally, there have been document reading apparatuses each using a CCD image sensor that has output terminals, including a combination of First and Last terminals, a combination of the First and Last terminals and odd-number (Odd) and even-number (Even) terminals. In each of the conventional document reading apparatuses, those output terminals are provided for each line.
Japanese Laid-Open Patent Application No. 6-14188, filed by the present applicant, discloses such a document reading apparatus in which gamma correction is performed separately on the odd-number (O) data and the even-number (E) data of image data that are read by an image sensor. After A/D conversion and shading correction, the O/E levels are adjusted by gamma correction.
In this manner, the differences between the odd-number (O) outputs and the even-number (E) outputs from output terminals are adjusted after the A/D conversion. Gamma tables are provided separately for the O outputs and the E outputs, and the OE outputs are adjusted at each level.
However, only integer correction can be performed by the method of performing gamma correction on digital data as disclosed in the above mentioned patent publication. Furthermore, the gradient in a region with a great non-linear inclination might be degraded by such a method.
This conventional correction method is effective for the O/E correction, but it is not effective enough with one of the latest CCDs that includes a terminal (First terminal) that sequentially outputs the first half of the pixels of an image, starting from the first pixel, and a terminal (Last terminal) that sequentially outputs the last half of the pixels, starting from the last pixel. With such a CCD, there is a boundary at the center between the First region and the Last region in each image, and an output difference (F/L difference) as small as 1 LSB stands out on the boundary.